Transcript Newton`s Laws Notes

Laws of Motion
Mrs. Wolfe
“If I have seen far, it is because I have stood on the
shoulders of giants.”
- Sir Isaac Newton
(referring to Galileo)
What is Force?
• Force
– a push or pull that one object exerts on another
– What forces are being exerted on the football?
Fkick
Fgrav
Draw a diagram showing the forces…
• A girl is hanging
from a bar held
from the ceiling by
two ropes.
Draw a diagram of …
• A flying squirrel
is gliding from a
tree to the ground.
Try one more …
• A piano is pushed
to the right across
the room.
Balanced & Unbalanced forces
• Which diagram(s) show BALANCED forces?
Balanced Forces
–Forces acting on
an object that are
opposite in
direction and
equal in size
–Causes no
change in
velocity
Time for a DEMO!
• UNBALANCED forces cause a
CHANGE IN VELOCITY
(acceleration)!
Which object(s) will
ACCELERATE?
Time for a WS!
Unbalanced Forces & Motion WS
The Big Misconception
Read The Big
Misconception
Handout!
Anna is right!!
• These are BALANCED FORCES
• No ACCELERATION
• That means, NO CHANGE IN VELOCITY!
• It could be moving, but it will keep moving at a
constant speed in the same direction.
• It could be at rest, but it will remain at rest.
Forces DO NOT CAUSE MOTION!
• FORCES CAUSE ACCELERATION!
NET FORCE
• Net Force =
OVERALL FORCE
• Force is measured
in Newtons (N).
• What is the NET
FORCE on each
object?
400 N
200 N
20 N
Got it? Time for a WS!
Force & Acceleration WS
Newton’s First Law of Motion
• An object at rest will remain at
rest and an object in motion will
continue moving at a constant
velocity unless acted upon by an
unbalanced force.
• Things are LAZY…they keep
doing what they’re already doing
unless an UNBALANCED force
causes a change!
The Law of Inertia
• Newton’s First Law of Motion
–“Law of Inertia”
• Inertia
–Laziness
–tendency of an object to resist
any change in its motion
–increases as mass increases
In a nutshell…
Demo:
• Magic Golf Balls
Demo:
• Magic Coins
More examples…
How fast are you going when your face
hits the pavement?
Time for a Lab!
• 1st Law of Motion Lab
Demo:
• Tossing a ball in a car (moving at
a constant velocity). Where does
it land?
Time for a WS!
• 1st Law of Motion WS
Newton’s Second Law of Motion
• Force = Mass x Acceleration
F: (N) m: (kg)
a: (m/s2)
1 N = 1 kg ·m/s2
F = ma
F
m a
Time for a WS!
• 2nd Law WS
Demo:
Time for a Lab!
• 2nd Law Lab
Demo:
Back to the BIG
MISCONCEPTION!
• Why does it take an UNBALANCED
force to cause an acceleration?
Because F = m • a
Fnet = 0
0=m•a
A=0
It can’t accelerate!
Demo: How does a CONSTANT
VELOCITY affect FORCE?
Newton’s Third Law of Motion
• When one object
exerts a force on
a second object,
the second object
exerts an equal
but opposite
force back on the
first.
Demo:
Problem:
–How can a horse
pull a cart if the cart
is pulling back on
the horse with an equal but
opposite force?
–Aren’t these “balanced forces”
resulting in no acceleration?
NO!!!
Answer:
–forces are equal and opposite
but act on different objects
–they are not “balanced
forces”
–Balanced forces act on the
SAME OBJECT
Demo:
Problem:
–A high-speed truck and a
BUG have a head-on
collision. The bug is
crushed, while nothing
happens to the truck. Do
they hit each other with
equal force?
Answer:
–Yes! Equal, but Opposite Forces!
–The force acting on the bug is
enough to crush the bug. The same
force acting on the truck isn’t
enough to hurt the truck.
Time for a WS!
• 3rd Law WS
Identify the Action-Reaction Force Pairs
Identify 2 Action-Reaction Force Pairs
Identify 5 Action-Reaction Force Pairs
Time for a Lab!
• 3rd Law Lab
Do you understand?
Time for a Review WS!
May the NET FORCE be with you!